1. Field of the Invention
This invention relates to a focal plane shutter for a camera. More particularly, it is a focal plane shutter of the type including two sets of thin lightshielding blades which are adapted to travel across a shutter aperture.
2. Description of the Prior Art
There is known a focal plane shutter comprising a plurality of thin light-shielding blades of which every two adjoining ones overlap each other, and which are successively movable to vary their overlapping width to thereby open and close a shutter aperture. When the blades spread over the shutter aperture to close it, every two adjoining blades are still required to maintain a minimum overlapping width to prevent any passage of light. In order to maintain the minimum effective overlapping width and yet reduce the size of the shutter, it has hitherto been necessary to increase the number of the blades. This increase has added to thc total weight of the blades. It has, however, become necessary to reduce the weight of the blades in order to realize a high-speed shutter. In order to reduce the weight of the blades, it has been usual practice to reduce their overlapping width, size, or number. This practice has, however, made it necessary to provide additional light-shielding members, or manufacture the blades with a higher degree of accuracy with respect to warpage or curvature, in order to prevent any passage of light through the shutter aperture when it is closed.
A focal plane shutter of the known construction is shown by way of example in FIGS. 3(a) and 3(b). It includes a base plate 1 having an aperture 1a through which light can be passed to expose a film, and a cover plate 2 lying in parallel to the base plate 1. The base plate 1 and the cover plate 2 define therebetween a space in which a plurality of shutter blades are movable to open and close the aperture 1a. A partition 11 is provided between the base plate 1 and the cover plate 2 for dividing the space therebetween into two section in which two sets of shutter blades are respectively mounted. The cover plate 2 and the partition 11 are substantially equal in shape to the base plate 1 and are supported thereon in a known way not shown. The cover plate 2 has an aperture 2a and the partition 11 also has an aperture 11a. The aperature 2a and 11a are equal in size to the aperature 1a of the base plate 1 and are axially aligned therewith.
The shutter blades consist of a set of blades 3 to 7 which are provided between the base plate 1 and the partition 11 movably for opening the aperature 1a for film exposure, and a set of blades 10 which are provided between the cover plate 2 and the partition 11 for closing the opening 1a. The blades 10 are substantially of the same construction with the blades 3 to 7, but are symmetrical thereto when viewed in a vertical plane.
A pair of arms 8 and 9 are rotatably supported on the base plate 1 by a pin 8a and a pin 9a, respectively. The blades 3 to 7 are rotatably connected to the arm 8 by pins 3b to 7b, respectively, and to the arm 9 by pins 3c to 7c, respectively. The pins 8a and 9a form a pair of parallel links with each of the other pairs of pins, i.e., 7b and 7c, 6b and 6c, 5b and 5c, 4b and 4c, and 3b and 3c.
The film 12 is positioned behind the cover plate 2. A photographic lens not shown is located in front of the base plate 1.
Every adjoining two of the blades 3 to 7 overlap each other to some extent, as shown in FIG. 3(a). The overlapping widths L.sub.1, L.sub.2, L.sub.3 and L.sub.4 of the blades 3 and 4, 4 and 5, 5 and 6, and 6 and 7, respectively, are substantially equal to one another. If one of the arms 8 and 9 is rotated, the blades are moved to open the aperature 1a, while maintaining their parallel relationship.